In recent years, there has been growing interest in the development of nonsteroidal modulators for steroid receptors for therapeutical use. It has been shown that nonsteroidal ligands can achieve better receptor selectivity and better physicochemical, pharmacokinetic and pharmacological properties. For androgen receptor (AR), nonsteroidal AR antagonists (antiandrogens), such as bicalutamide, are now used clinically to counteract the undesirable actions of excessive androgens, particularly in the treatment of prostate cancer.
Androgens such as testosterone and its conversion product dihydrotestosterone (DHT), functioning through the AR, are essential for the initiation and progression of prostate cancer. Thus, treatment of advanced prostate cancer involves androgen-ablation therapies, such as surgical castration or hormonal manipulation using gonadotropin-releasing hormone (GnRH) agonists, anti-androgens or both. Although such therapies initially lead to disease regression, eventually all patients progress to a castration resistant late stage that is refractory to current therapies. Castration-resistant prostate cancer (CRPC) is often associated with increased levels of AR. First generation anti-androgens such as bicalutamide display agonistic properties in cells engineered to express higher AR levels. In vitro and in vivo, increased AR expression has been shown to confer resistance of prostate cancer cell lines to anti-androgen therapy. To overcome resistance problems, second generation anti-androgens that retain antagonism in cells expressing excess AR may have utility in the treatment of CRPC.
Prostate cancer can be also treated by inhibiting the biosynthesis of androgens. In the testes and adrenal glands, the last step in the biosynthesis of testosterone involves two key reactions, which are both catalyzed by a single enzyme, the cytochrome P450 monooxygenase 17α-hydroxylase/17,20-lyase (CYP17). Ketoconazole, an antifungal agent, which is also a modest CYP17 inhibitor has been used clinically for the treatment of prostate cancer. It has been reported that careful scheduling of treatment can produce prolonged responses in otherwise hormone-refractory prostate cancer patients. Although ketoconazole has been withdrawn from the use because of liver toxicity and other side effects, this suggests that more potent and selective inhibitors of CYP17 could provide useful agents for treating prostate cancer, even in advanced stages and in some patients who may appear to be hormone refractory.
Recently, a potent CYP17 inhibitor abiraterone was approved in combination with prednisone for the treatment of CRPC. Abiraterone has been reported to increase survival and to delay clinical decline and initiation of chemotherapy in CRPC patients who have had no prior chemotherapy.